Re: [PATCH] futexs: fix infinite loop in get_futex_key on huge page

Ingo Molnar
2009-07-11T08:20:06Z

* Sonny Rao <sonnyrao@us.ibm.com> wrote:

> get_futex_key() can infinitely loop if it is called on a virtual
> address that is within a huge page but not aligned to the
> beginning of that page. The call to get_user_pages_fast will
> return the struct page for a sub-page within the huge page and the
> check for page->mapping will always fail.
>
> The fix is to call compound_head on the page before checking that
> it's mapped.
>
> Signed-off-by: Sonny Rao <sonnyrao@us.ibm.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: stable@kernel.org
>
> Index: linux-2.6.31-rc2/kernel/futex.c
> ===================================================================
> --- linux-2.6.31-rc2.orig/kernel/futex.c 2009-07-10 17:45:46.181084475 -0500
> +++ linux-2.6.31-rc2/kernel/futex.c 2009-07-10 17:46:47.345084062 -0500
> @@ -247,6 +247,7 @@
> if (err < 0)
> return err;
>
> + page = compound_head(page);
> lock_page(page);
> if (!page->mapping) {
> unlock_page(page);

Nice catch! Applied it to tip:core/urgent - Thomas, do you agree
with the fix?

Ingo